The over-bump applied resin wafer-level underfill process: Process, material and reliability

The over bump applied resin (OBAR) process is a wafer-level underfill (WLUF) process in which a filled resin is applied over the bumps of a wafer and, dried. The wafer is diced into coated chips which are aligned and joined to a substrate resulting in an underfilled flip chip package. This process has been evaluated by IBM on several test vehicles in close cooperation with Henkel (formerly Abelstik) who developed a material specifically to fit this process. The critical steps to make this technology work are alignment of OBAR coated chip to a substrate, elimination of significant voids, formation of a fillet with appropriate shape and size, fluxing and solder joining. The reliability of the material was evaluated after capping and BGA (Ball Grid Array) attach through JEDEC level 3 preconditioning followed by DTC (deep thermal cycling), T&H (temperature and humidity), and HTS (high temperature storage). While some improvements are still needed, the OBAR process has been shown to be a viable alternative to capillary underfill application.